METHOD AND SYSTEM FOR CHARACTERIZING AND VISUALIZING ELECTROMAGNETIC TRACKING ERRORS
A calibration/surgical tool (90, 160) includes an electromagnetic sensor array (30) of two or more electromagnetic sensors in a known geometrical configuration. Electromagnetic tracking errors are characterized by a mapping of pre-operative absolute and relative errors based on a movement of a calibrated calibration/surgical tool (90, 160) through a pre-operative electromagnetic field. Using statistical mapping, a desired absolute error field (46) is measured either in the clinic as the part of daily quality control checks, or before the patient comes in or in vivo. A resulting error field (46) may be displayed to the physician to provide clear visual feedback about measurement confidence or reliability of localization estimates of the absolute errors in electromagnetic tracking.
Latest KONINKLIJKE PHILIPS ELECTRONICS N.V. Patents:
- METHOD AND ADJUSTMENT SYSTEM FOR ADJUSTING SUPPLY POWERS FOR SOURCES OF ARTIFICIAL LIGHT
- BODY ILLUMINATION SYSTEM USING BLUE LIGHT
- System and method for extracting physiological information from remotely detected electromagnetic radiation
- Device, system and method for verifying the authenticity integrity and/or physical condition of an item
- Barcode scanning device for determining a physiological quantity of a patient
The present invention generally relates to electromagnetic tracking systems for clinical procedures. The present invention specifically relates to a characterization and a visualization of electromagnetic tracking errors within electromagnetic fields.
Electromagnetic tracking systems are often used for real time navigation of surgical tools in an Image Guided Therapy (“IGT”) system. Electromagnetic tracking systems are however very sensitive to electromagnetic field distortions. These distortions arise in a clinical environment due to a presence of a ferromagnetic interventional apparatus or other metallic medical equipment. In the presence of a field distortion, electromagnetic tracking measurements result in non-uniform, complex error distributions that impact the ability of a physician to navigate the surgical tools for therapy delivery with accuracy and precision.
The ability to rapidly characterize and map any potential errors in the interventional workspace plays an important role in providing physicians with information about whether the desired location of treatment can be targeted with confidence. Thus, attempts have been made to characterize and correct electromagnetic tracking errors based on pre-procedural calibration techniques. For example, a pre-operative static map of the whole electromagnetic field may be generated for use as a look-up table to correct electromagnetic tracking errors at any given position/orientation of each electromagnetic sensor. By further example, optical markers exclusively or in conjunction with electromagnetic sensors may be used. However, clinical environments dynamically change during a procedure making pre-procedural calibration measurements difficult to apply intra-procedurally.
The present invention detects and characterizes electromagnetic tracking errors by mapping absolute and relative errors pre-procedurally within an electromagnetic field. To this end, a calibration tool or a surgical tool includes an electromagnetic sensor array of two or more electromagnetic sensors in a known geometrical configuration. This approach is distinct from other calibration tool designs in that error characterization is derived solely from electromagnetic sensor measurements rather than by using reference measurements from optical markers or other sensing techniques. The relative error is measured as the difference between the known geometry and the electromagnetically sensed one in real time. Using statistical mapping, a desired absolute error field space is measured either in the clinic as the part of daily quality control checks, or before the patient comes in, or in vivo. A resulting error field displayed to the physician provides clear visual feedback about measurement confidence or reliability of localization estimates of the absolute errors in electromagnetic tracking.
One form of the present invention is an electromagnetic error tracking method having a calibration stage, a pre-operative stage and an intra-operative stage. For purposes of the present invention, the term “calibration” as used herein is broadly defined to describe any activity occurring or related to a calibration of an electromagnetic sensor array, the term “pre-operative” as used herein is broadly defined to describe any activity occurring or related an application of calibration data of the electromagnetic sensor for purposes of generating an error map as further described herein, and the term “intra-operative” as used herein is broadly defined to describe as any activity occurring or related to an application of the pre-operative error map for purposes of generating an absolute error field as further described herein.
The calibration stage involves a design of a calibration/surgical tool having a known geometrical configuration of an electromagnetic sensor array of two (2) or more electromagnetic sensors (e.g., coils). The electromagnetic sensor array is disposed within a calibration electromagnetic field, and a calibrated distance between one or more electromagnetic sensors pairs is measured from a sensing of the electromagnetic sensor array within the calibration electromagnetic field. For purposes of the present invention, the term “electromagnetic sensor pair” is broadly defined herein as any two (2) electromagnetic sensors of the electromagnetic sensor array designated as a pair for purposes of calibrating the electromagnetic sensor array and for computing relative errors as further described herein.
The pre-operative stage involves a controlled movement of the electromagnetic sensor array within a pre-operative electromagnetic field between numerous measurement positions. For each electromagnetic sensor, a pre-operative absolute error for the electromagnetic sensor is measured at each measurement position of the electromagnetic sensor with the each pre-operative absolute error for the electromagnetic sensor being an absolute differential between a measurement position and a sensed position of the electromagnetic sensor within the pre-operative electromagnetic field. Also for each electromagnetic sensor pair, a pre-operative relative error is measured at each measurement position with each pre-operative relative error being an absolute differential between a calibrated distance between the electromagnetic sensor pair and a sensed distance between the electromagnetic sensor pair within the pre-operative electromagnetic field. A pre-operative error map is generated from a statistical relationship between the pre-operative absolute errors and the pre-operative relative errors.
The intra-operative stage involves a controlled movement of the electromagnetic sensor array within an intra-operative electromagnetic field between numerous estimation positions. For each electromagnetic sensor pair, an intra-operative relative error is measured at each estimation position with each intra-operative relative error being an absolute differential between a calibrated distance between an electromagnetic sensor pair and a sensed distance between the electromagnetic sensor pair within the intra-operative electromagnetic field. For each estimation position, an intra-operative absolute error is estimated from a plotting of the corresponding intra-operative relative error within the pre-operative error map of the pre-operative absolute errors and the pre-operative relative errors. The intra-operative stage may further involve feedback (e.g., visual, audio and/or tactile) representative of the estimated intra-operative absolute errors. In one embodiment, an image of an object within the intra-operative electromagnetic field (e.g., an anatomical region of a body) may be integrated with a visual feedback of an intra-operative absolute error field having one or more reliable zones indicative of one or more undistorted areas in the intra-operative electromagnetic field and/or one or more unreliable zones indicative of one or more distorted areas in the intra-operative electromagnetic field. The intra-operative absolute error field may be derived from a comparison of the estimated intra-operative absolute errors to a reliability threshold.
A second form of the present invention is an electromagnetic tracking system of the present invention employing a tool (e.g., calibration or surgical) including the electromagnetic sensor array and a data processor for executing one or more of the calibration stage, the pre-operative stage and the intra-operative stage.
The foregoing forms and other forms of the present invention as well as various features and advantages of the present invention will become further apparent from the following detailed description of various embodiments of the present invention read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof.
One definition of an absolute error for an electromagnetic sensor the art is a registration error between a tracking of an electromagnetic sensor and a reference navigation system (e.g., a robot or an optical tracking system.) The present invention is premised on a measurement of relative errors between two (2) electromagnetic sensors having correlated absolute errors.
Specifically,
For spatial volume 20, an electromagnetic sensor array 30 (e.g., coils) having a known geometrical configuration is disposed within the calibration electromagnetic field that is generated in a clean room. A sensing of the coils facilitates a measurement of a calibrated distance between pairs of electromagnetic sensors. For example,
Referring back to
For example,
Referring back to
For the spatial volume 23, electromagnetic sensor array 30 is disposed within the intra-operative electromagnetic field that is generated in a clinical environment having ferromagnetic interventional equipment or other metallic medical equipment (e.g., an X-ray system). For each electromagnetic sensor pair, an intra-operative relative error is measured as the electromagnetic sensor array 30 is moved to numerous estimation positions with each intra-operative relative error being an absolute differential between a calibrated distance of the electromagnetic sensor pair and a sensed distance between the electromagnetic sensor pair within the intra-operative electromagnetic field. For each estimation position, an intra-operative absolute error is estimated from a plotting of the corresponding intra-operative relative error within the pre-operative error map 42.
For example, referring to
By further example, as shown in
A dataset 44 of estimated intra-operative absolute errors is used to generate feedback (e.g., visual, audio and/or tactile) representative of the estimated intra-operative absolute errors. As shown in
Various embodiments of the present invention will now be described herein in connection with
Calibration stage 60 includes a sensor array designer 61 for defining a geometrical configuration of electromagnetic sensors for a calibration tool or a surgical tool. In practice, the geometrical configuration may have any positioning and orientation of each electromagnetic sensor suitable within the array suitable for tracking the electromagnetic sensors in a clinical and clinical environment. In one exemplary embodiment as shown in
In order to establish, a statistical relation between relative errors and absolute errors as will be further explained herein, the electromagnetic measurements from electromagnetic sensor pair 93(1) and 93(2) for example may be considered as two random variables. As such, the relative error between electromagnetic sensor pair 93(1) and 93(2) is the difference between two random variables AES1 and AES2, which are the errors in world coordinates, which are referred to as absolute errors at electromagnetic sensors 93(1) and 93(2) respectively. Assuming that the probability density functions for absolute errors AES1 and AES2 are identically distributed with zero means in accordance with the follow equation:
In addition, the variance , where and are the standard deviation of AES1 and correlation factor between AES1 and AES2, respectively. Whenever the sensors 93(1) and 93(2) are physically close to each other, they are highly linearly correlated in the positive direction, thereby driving down the variability in relative error RE, , to zero and making the relative error RE homogenous.
For the present invention, the smaller variability of relative error RE is used to estimate the value of absolute tracking AE. In order for degree of variation to be minimal, the above equation [1] imposes two conditions. First, the two sensors 93(1) and 93(2) should not be far apart from each other, and second, the distance of the sensors 93(1) and 93(2) from electromagnetic field generator (not shown) should not be large. If the first condition is violated, then the correlation coefficient will not be close to one (1), thereby increasing the variation in consecutive relative error RE measurements. If the second condition is violated, will be high, in turn increasing the variance of relative error RE. The relationship between these three variables— and is shown in the correlation graph 100 of
Calibration stage 60 further includes a sensor array calibrator 62 for measuring a calibration distance CD between pairs of electromagnetic sensors. For example, as shown in
Pre-operative stage 70 employs an absolute error measurer 71 for measuring pre-operative absolute errors and computing pre-operative relative errors as previously described herein for
Pre-operative stage 70 further employs an error mapper 72 for mapping the pre-operative absolute errors and the pre-operative relative errors for each position. The error mapping may be a derived from a statistical relationship between the pre-operative absolute errors and the pre-operative relative errors. In one exemplary embodiment, each relative error RE may be mapped to a probable absolute error observation AE at a given measurement location. Given a specific value of relative error RE, the minimum mean square error estimator of absolute error AE is the expected value of the conditional probability, , where f is the conditional probability density function of absolute error AE given relative error RE. Therefore, if the joint probability function of absolute error AE and relative error
RE is empirically estimated, then the statistics of the absolute errors AE can be estimated using the relative errors RE. This can be done by collecting large samples of data and observing the relationship between absolute and relative errors.
In estimating the joint probability, it is important to make sure that the measurements are made under different types of expected/realistic electromagnetic distorted environments. In one embodiment, data is collected from both highly distorted and minimally distorted environments in vicinity of an X-ray gantry or CT scanner. Six (6) sets of data is collected, each dataset including more than 13,000 points. Within each dataset, absolute errors are measured. The union of the six (6) datasets is normalized to get a probability mapping of absolute error AE versus relative error RE.
In practice, pre-operative stage 70 may be run within any clinical environment having electromagnetic distortions. For example, as shown in
Referring again to
In one exemplary embodiment, an estimate of absolute error AE for the workspace can be measured by taking relative error RE measurements at N different locations in the workspace. For each location, an estimate of absolute error AE can be measured using the 2D histogram by executing the following steps. The first step involves a measurement of the marginal pdf of P(RE) by summing along the rows of the 2D histogram and normalizing. The second step, for each measurement of relative error RE, involves a consideration corresponding column of the histogram and a weighing of the probability values of that column by 1/P(RE) to get probability distribution P(AE|RE). The third step involves a measurement of E(AE|RE) by computing the mean of this conditional probability distribution. The fourth step involves a repeating of the first three (3) steps for N different measurements of relative error RE, and the mean (E(AE|RE) is measured. This mean reflects the estimated value of AE given N different RE measurements. The fifth step involves a measurement of a standard deviation, from N measurements of relative error RE to get an estimate of confidence in the estimation of absolute error AE. For each set of N measurements, the result from the fourth step may be used to create an intra-operative absolute error field displayed to the physician as a part of this invention to provide a clear visual feedback about the measurement confidence. In addition, the result from the fifth step provides a confidence of the error estimates.
In practice, intra-operative stage 80 may be run within any clinical environment having electromagnetic distortions. For example, as shown in FIGS, 12 and 13, calibration tool 90 is moved in a controlled manner by a position system (not shown) or medical professional (not shown) through an electromagnetic field (not shown) generated by an electromagnetic field generator 111 relative to an imaging modality (not shown)(e.g., an X-ray machine that distorts the electromagnetic field causing absolute errors for calibration tool 90). A patient 140 as shown may or may not be present. Relative error measurer 81 computes intra-operative relative errors for electromagnetic sensors at each estimation position, and absolute error estimator 82 estimates intra-operative absolute errors derived from a plotting of the intra-operative relative errors within the pre-operative error map 42. Responsive to a comparison of estimated intra-operative absolute errors 44 to reliability threshold 45, absolute error field generator 84 generates a visual feedback in the form of an absolute error field 46 having one or more reliable zones and/or one or more unreliable zones.
More particularly, an electromagnetic sensor array of a know geometrical configuration may be incorporated into surgical tools (e.g., catheters or needles) for use in detecting reliable zones of operation within an anatomy of interest, such as, for example, an in vivo deployment directly within the tissue of interest. A characterization of EM tracking errors may be performed “live”, rather than requiring a separate step immediately prior to patient preparation as with a calibration tool, which streamlines the surgical workflow significantly.
The modules include software, hardware and/or firmware for executing various processes for characterizing and visualizing electromagnetic tracking errors in accordance with the present invention. To this end, data processor 210 includes one or more processors of any known type(s) and one more memories of any known type(s) to operate the modules. In practice, the modules may be individual modules within respective data processors as shown, or one or more of the modules may integrated within respective data processors. Furthermore, data processors 210 and 220 may be individual data processors as shown or integrated within one machine. Alternatively, intra-operative modules 224-226 may be installed within a different data processor than data processor 220 with calibration data 40 and error map 42 (
In practice, electromagnetic sensor array 201 may be rotated and/or pivoted at each measurement position of pre-operative stage 70 and/or each estimation position of intra-operative stage 80 to further enhance the for characterization and visualization of electromagnetic tracking errors in accordance with the present invention.
While various embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that the embodiments of the present invention as described herein are illustrative, and various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention without departing from its central scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention includes all embodiments falling within the scope of the appended claims.
Claims
1. An electromagnetic tracking system (200), comprising:
- an electromagnetic sensor array (30) including at least two electromagnetic sensors arranged in a known geometrical configuration; and
- a data processor (220) in electrical communication with the electromagnetic sensor array (30) to receive signals indicative of a sensing of an electromagnetic field by the electromagnetic sensors, wherein the data processor (220) is operable to compute intra-operative relative errors (43) responsive to a movement of the electromagnetic sensor array (30) to numerous estimation positions within an intra-operative electromagnetic field, wherein each intra-operative relative error (43) is an absolute differential between a calibrated distance (40) of an electromagnetic sensor pair and a sensed distance between the electromagnetic sensor pair within the intra-operative electromagnetic field, wherein the data processor (220) is further operable to estimate intra-operative absolute errors (44) responsive to a plotting of the intra-operative relative errors (43) within a pre-operative error map (42) representative of a statistical relationship between pre-operative absolute errors (41) and pre-operative relative errors (41) derived from a movement of the electromagnetic sensor array (30) to numerous measurement positions within a pre-operative electromagnetic field, wherein each intra-operative absolute error (44) is an absolute differential between an estimation position and a sensed position of an electromagnetic sensor within the intra-operative electromagnetic field, wherein each pre-operative absolute error (41) is an absolute differential between a measurement position and a sensed position of an electromagnetic sensor within the pre-operative electromagnetic field, and wherein each pre-operative relative error (41) is an absolute differential between a calibrated distance (40) and a sensed distance between an electromagnetic sensor pair within the pre-operative electromagnetic field.
2. The electromagnetic tracking system (200) of claim 1,
- wherein the data processor (220) is operable to provide feedback representative of an estimation of the intra-operative absolute errors (44), and
- wherein the feedback includes at least one of a visual feedback, an audio feedback and a tactile feedback.
3. The electromagnetic tracking system (200) of claim 2, further comprising:
- an imaging system (203) in electrical communication with the data processor (220) to visually display the visual feedback.
4. The electromagnetic tracking system (200) of claim 1,
- wherein the data processor (220) is further operable to generate an intra-operative absolute error field (46) responsive to an estimation of the intra-operative absolute errors (44),
- wherein the intra-operative absolute error field (46) includes at least one of a reliable zone (24a) and an unreliable zone (24b),
- wherein the reliable zone (24a) is indicative of an undistorted area of the intra-operative electromagnetic field, and
- wherein the unreliable zone (24b) is indicative of a distorted area of the intra-operative electromagnetic field.
5. The electromagnetic tracking system (200) of claim 4, further comprising:
- an imaging system (203) in electrical communication with the data processor (220) to integrate the intra-operative absolute error field (46) and an image of an object within the intra-operative electromagnetic field.
6. The electromagnetic tracking system (200) of claim 1,
- wherein the data processor (220) is further operable to generate an intra-operative absolute error field (46) responsive to a comparison of a reliability threshold (45) to an estimation of the intra-operative absolute errors (44),
- wherein the intra-operative absolute error field (46) including at least one of a reliable zone (24a) and an unreliable zone (24b),
- wherein the reliable zone (24a) is indicative of an undistorted area of the intra-operative electromagnetic field, and
- wherein the unreliable zone (24b) being indicative of a distorted area of the intra-operative electromagnetic field.
7. The electromagnetic tracking system (200) of claim 5, further comprising:
- an imaging system (203) in electrical communication with the data processor (220) to visually display an integration of the intra-operative absolute error field (46) and an image of an object within the intra-operative electromagnetic field.
8. The electromagnetic tracking system (200) of claim 1, further comprising:
- a calibration tool (90), wherein the electromagnetic sensor array (30) is incorporated in the calibration tool (90).
9. The electromagnetic tracking system (200) of claim 1, further comprising:
- a surgical tool, wherein the electromagnetic sensor array (30) is incorporated in the surgical tool.
10. The electromagnetic tracking system (200) of claim 9,
- wherein the surgical tool (160) includes a sheath (163); and
- wherein the electromagnetic sensor array (30) is patterned on an electronic substrate bonded to a surface of the sheath (163) and covered with a barrier coating.
11. The electromagnetic tracking system (200) of claim 9,
- wherein the surgical tool includes a mandrin (170); and
- wherein the electromagnetic sensor array is patterned on an electronic substrate bonded to a surface of the mandarin (170) and covered with a barrier coating.
12. The electromagnetic tracking system (200) of claim 9,
- wherein the surgical tool (90) includes a needle (170); and
- wherein the electromagnetic sensor array (30) is patterned on an electronic substrate bonded to a surface of the needle (170) and covered with a barrier coating.
13. The electromagnetic tracking system (200) of claim 9,
- wherein the surgical tool includes a deflectable balloon (185); and
- wherein the electromagnetic sensor array is patterned on an electronic substrate bonded to a surface of the deflectable balloon (185) and covered with a barrier coating.
14. The electromagnetic tracking system (200) of claim 9,
- wherein the surgical tool includes a mesh (192); and
- wherein the electromagnetic sensor array (30) is patterned on an electronic substrate bonded to a surface of the mesh (192) and covered with a barrier coating.
15. The electromagnetic tracking system (200) of claim 1, further comprising:
- an electromagnetic field generator (202) operable to generate at least one of the pre-operative electromagnetic field and the intra-operative electromagnetic field.
16. An electromagnetic tracking method for a tool (90) including a known geometrical configuration of an electromagnetic sensor array (30) of two or more electromagnetic sensors, the method comprising:
- a measurement of a calibrated distance (40) between at least one electromagnetic sensor pair responsive to the electromagnetic sensor array (30) being within a calibration electromagnetic field;
- a controlled movement of the electromagnetic sensor array (30) within a pre-operative electromagnetic field to various measurement positions; for each electromagnetic sensor, a measurement of a pre-operative absolute error (41) at each measurement position of the electromagnetic sensor, wherein each pre-operative absolute error (41) is an absolute differential between a measurement position and a sensed position of the electromagnetic sensor within the pre-operative electromagnetic field; and for each pairing of electromagnetic sensors, a measurement of a pre-operative relative error (41) at each measurement position, wherein each pre-operative relative error (41) is an absolute differential between a calibrated distance (40) of an electromagnetic sensor paring and a sensed distance of the electromagnetic sensor pair within the pre-operative electromagnetic field.
17. The electromagnetic tracking method of claim 16, further comprising:
- a generation of a pre-operative error map (42) derived from a statistical relationship between the pre-operative absolute errors (41) and the pre-operative relative errors (41).
18. The electromagnetic tracking method of claim 17, further comprising:
- a controlled movement of the electromagnetic sensor array (30) within an intra-operative electromagnetic field to various estimation positions;
- for each electromagnetic sensor pair, a measurement of an intra-operative relative error (43) at each estimation position, wherein each intra-operative relative error (43) is an absolute differential between a calibrated distance (40) of an electromagnetic sensor pair and a sensed distance of the electromagnetic sensor pair within the intra-operative electromagnetic field; and
- for each estimation position, an estimation of an intra-operative absolute error (44) derived from a plotting of the intra-operative relative errors (43) within the pre-operative error map (42).
19. The electromagnetic tracking method of claim 18, further comprising:
- a generation of feedback representative of an estimation of the intra-operative absolute errors (44), wherein the feedback includes at least one of a visual feedback, an audio feedback and a tactile feedback.
20. The electromagnetic tracking system (200) of claim 18, further comprising:
- a generation of an intra-operative absolute error field (46) responsive to an estimation of the intra-operative absolute errors (44), wherein the intra-operative absolute error field (46) includes at least one of a reliable zone (24a) and an unreliable zone (24b), wherein the reliable zone (24a) is indicative an undistorted electromagnetic area of the intra-operative electromagnetic field, and wherein the unreliable zone (24b) is indicative of a distorted electromagnetic area of the intra-operative electromagnetic field.
Type: Application
Filed: Feb 21, 2011
Publication Date: Dec 20, 2012
Patent Grant number: 9165114
Applicant: KONINKLIJKE PHILIPS ELECTRONICS N.V. (EINDHOVEN)
Inventors: Ameet Kumar Jain (New York, NY), Mohammad Babak Matinfar (Baltimore, MD), Raymond Chan (San Diego, CA), Vijay Parthasarthy (Tarrytown, NY), Douglas A. Stanton (Ossining, NY)
Application Number: 13/582,062
International Classification: A61B 5/055 (20060101);